Method of forming and heat treating a composite wire



United States Patent Inventors Paul O. Hagarman and Paul A. Dion, North Attleboro, Massachusetts Appl. No. 640,870

Filed May 24, 1967 Patented Nov. 3, 1970 Assignee Texas Instruments Incorporated Dallas, Texas a corporation of Delaware METHOD OF FORMING AND HEAT TREATING A COMPOSITE WIRE 5 Claims, 5 Drawing Figs.

U.S. Cl 148/1 1.5, 148/13, 148/27 Int. Cl. C22f l/00 Field of Search... 148/154,

[56] References Cited UNITED STATES PATENTS 1,285,887 11/1918 Alexander 148/154 2,876,137 3/1959 Drummond 117/107 3,261,724 7/1966 Ulam 148/1 1.5 3,335,260 8/1967 Ferschl.... 148/154 3,366,476 1/1968 Jagaciak 148/11.5

Primary Examiner-Byland Bizot Attorneys- Harold Levine, Edward J. Connors, Jr., John A.

l-Iaug and James P. Mc Andrews ABSTRACT: A composite wire having a copper cladding on an aluminum core is provided with selected strength and deformation characteristics by closely regulated, high temperature, short duration, inline resistance annealing which provides the wire core and cladding with selected tempers without significant formation of embrittling intermetallic compounds at the interface between the wire core and cladding.

Patented Nov. 3, 1970 CONiiUTIVITY COMPARABLE CGPPER WIRE TENSILE. .srnsnsm KPsl 100 1000 TIME UNITS 24 p 2 22 30 Q2 2; 20 Q ,8 I5 s; I l0 2 l6 w J 3 u .22 .23 .24 .25 .26 .27 .29 .30

HEAT INPUT, JOUL-ES/CMF Invmztams:

Paul 0. .Hayarm an,

METHOD OF FORMING AND HEAT TREATING A COMPOSITE WIRE Composite wire materials embodying cores of aluminum and claddings of copper are conventionally batch annealed for long periods at low temperatures on the order of 475F. In this way, the wire cores and claddings are each substantially annealed without the formation of embrittling intermetallic compounds at the interface between the wire cores and claddings, such intermetallics tending to form quite rapidly between copper and aluminum materials at temperatures on the order of 650F. This batch annealing process is relatively slow and expensive and fine wire has a tendency to weld to adjacent turns when exposed to the necessary batch annealing cycle. Such an intermittent weld condition renders the wire useless.

ln-line resistance annealing, wherein wire is heated by passing electrical current through a length of the wire as the wire leaves an apparatus in which it has been drawn, has been used for some time in annealing wires formed of monolithic copper. In this annealing process, the copper is rapidly heated to a high annealing temperature on the order of l200F.- I500F. for a brief period for substantially fully annealing the copper. In-line resistance annealing has not previously been used in annealing composite wires such as copper-clad aluminum wire because lack of uniformity in the relative thickness of the wire core and cladding materials have resulted in irregular heating and annealing and braking of the composite wires. Further, the temperatures required for inline resistance annealing were known to substantially exceed the temperatures at which embrittling intermetallic compounds are formed at the interfaces between the cores and claddings of the composite copper-clad aluminum wires.

It is an object of this invention to provide novel and improved composite wire materials; to provide such materials having selected, improved strength and deformation characteristics; to provide such wire materials having aluminum cores and copper claddings in which the wire cores and claddings have different tempers; and to provide such copperclad aluminum wires which are substantially free of embrittling intermetallic compounds at the interfaces between the cores and cladding.

It is also an object of this invention to provide novel and improved methods for regulating the strength and deformation characteristics of composite wire materials; to provide novel and improved methods for annealing composite wire materials; to provide such annealing methods for furnishing the cores and claddings of composite wire materials with selected different tempers; to provide such methods for annealing copperclad aluminum wires without the formation of embrittling intermetallic compounds at the interfaces between the wire cores and claddings; and to provide such annealing methods which are simple and inexpensive to perform.

In this regard, it has been found that certain recently developed methods for forming copper-clad aluminum wires provide composite wires in which the cores and claddings have sufficient uniformity of thickness, and retain this uniformity during subsequent drawing, to permit precise and consistently reproducible control of resistance heating of the composite wires. Further, and most important, it has been found that the rate of recrystallization of copper and aluminum materials at high temperatures significantly exceeds the rate at which embrittling intermetallic compounds are formed at the interface between the composite wire cores and claddings. In addition, it has been noted that the rate of recrystallization of the copper material in the wire significantly exceeds the rate of recrystallization of aluminum at selected temperatures. It has also been found that the differences in resistance of the wire core and cladding materials, as well as the skin effect" inherent in electrical current transmission, result in different rates of heating of the wire core and cladding in composite wires.

Briefly described, the novel and improved method of this invention comprises the stepsof directing electrical current through a length of copper-clad aluminum wire of uniform core and cladding thickness, preferably as the wire is led away from an apparatus in which the wire has been drawn, for resistance-annealing the wire, and regulating the intensity and duration of the electrical current directed through the wire to provide selected, high annealing temperatures of very short duration so that the wire core and cladding are annealed to selected extents before embrittling intermetallic compounds are formed to any significant extent at the interface between the wire core and cladding. Preferably, the composite wire is moved through a cooling liquid bath immediately after annealing for rapidly cooling the wire to prevent growth of intermetallic compounds. In one embodiment of the method of this invention, the intensity and duration of the electrical current are regulated for substantially annealing both the wire core and cladding without the formation of embrittling intermetallic compounds at the interface between the wire core and cladding. In this way, a composite wire is provided with tensile strength and elongation characteristics comparable to batch annealed composite wires. This composite wire provided by this method of this invention is of substantially lower cost than comparable composite wires produced by previously known techniques. In another embodiment of the method of this invention, the intensity and duration of the electrical current are regulatedfor.substantially fully annealing only the coppercladding of the wire without the formation of embrittling intermetallic compounds at the wire core and cladding interface. In this way, a composite wire is provided with improved tensile and yield strengths and with slightly reduced elongation and bending deformation characteristics.

Other objects, advantages and details of this invention appear in the following detailed description of preferred embodiments of the invention, this description referring to the drawings in which:

FIG. 1 is a diagrammatic view illustrating the method of this invention;

FIG. 2 is a side elevation view, partly in section, illustrating the composite wire of this invention;

FIG. 3 is a schematic view illustrating the resistance heating characteristics of the composite wire of FIG. 2;

FIG. 4 is a graph illustrating annealing characteristics of composite wires; and

FIG. 5 is agraph illustrating the strength and deformation characteristics of the composite wire of this invention.

Referring to the drawings, FIG. 1 illustrates the method of this invention in which copper-clad aluminum wire is subjected to closely regulated, high temperature, short duration, in-line resistance annealing for providing the wire core and cladding with selected tempers without significant formation of embrittling intermetallic compounds at the wire core and cladding interface. The composite copper-clad aluminum wire to be. subjected to this resistance annealing preferably has core and cladding thicknesses which are substantially uniform, preferably within a tolerance of plus or minus about 5 percent of the desired thickness, to assure uniform wire annealing without breaking of the wire. Such uniformly proportioned composite wires can be formed, for example, in the apparatus described in the commonly-owned copending application,

Ser. No. 607,254 entitled Manufacture of Clad Wire and the Like" filed Jan. 4, 1967 now U.S. Pat. No. 3,444,603, in the name of Paul A. Dion and Arthur J. Thomson.

As shown in FIG. 1, 10 indicates any conventional apparatus for continuously drawing copper-clad aluminum wire 12 for reducing the wire to selected diameter. The wire core 14 of aluminum and wire cladding 16 of copper (see FIG. 2) are solid-phase metallurgically bonded together at an interface 18 between the core and cladding in conventional manner. As the drawing apparatus 10 can be of any conventional type, it is not further described herein and it will be understood that the wire 12 is continuously led from the drawing zone or apparatus 10 immediately after being drawn to selected diameter and is moved at a substantially uniform rate by any conventional means in the direction 20 to be wound on a takeup reel (not shown) in the conventional manner. Aswill be understood, the wire materials are in work-hardened condition leaving the drawing apparatus 10.

In accordance with this invention, a pair of contacts 22 and 24 are slidably engaged with the composite wire 12 as the wire is advanced from the apparatus 10, the contacts being connected to any suitable power source, preferably an alternating current source, as indicated by the terminals 26 and 28 in FIG. 1. In this way, electrical current is continuously directed through the length of composite wire 12 located between the contacts 22 and 24 for electrically heating the length of wire moving between the contacts. A rheostat 30 is connected in series with a voltmeter 32 between the contact 22 and terminal 26 to permit close regulation of the current directed through the length of composite wire. An ammeter 34 is preferably arranged in parallel with the voltmeter to assure precise current regulation. The spacing of the contacts 22 and 24 and the speed at which the composite wire 12 is moved in the direction are preferably adjustable relative to each other for determining the time during which the composite wire is resistance heated. Similarly, the rheostat is adjusted for regulating current flow through the wire as indicated by the voltmeter and ammeter. Preferably the composite wire 12 is then moved through a bath of cooling liquid as indicated at 36 for rapidly cooling the wire after annealing.

It has been found that, when the composite wire 12 is heated to temperatures up to about approximately 650--700 F., the rate of recrystallization or annealing of the wire materials occurs at a rate which is relatively slow as compared to the rate of formation of intermetallic compounds between the copper and aluminum wire materials. However, when the composite copper-aluminum wire is heated to temperatures between about 800F. and 1050F., it is found that the rate of recrystallization of the copper and aluminum materials becomes very advantageous as compared with the rate of in termetallic formation. This favorable rate differential continues only for a relatively brief period during initial heating of the wire.

For example, when copper-clad aluminum wire 0.025 inches in diameter embodying 20 percent copper by volume is heated to the noted temperatures, a marked increase in conductivity of the wire is noted during the initial heating of the wire only when the wire is heated at temperatures of 800F. or higher. In this regard, note that recrystallization or annealing of composite copper-clad aluminum wire tends to enhance conductivity of the wire whereas the formation of embrittling intermetallic compounds in the wire tends to decrease wire conductivity. That is, as indicated by curve a in FIG. 4, when the described 0.025 inch diameter wire is heated to a temperature of about 700F. it can be seen that the conductivity of the wire (expressed in terms of percentage of conductivity relative to copper wire of comparable diameter) does not show any significant increase, thereby indicating that the recrystallization of the wire materials and intermetallic formation are occurring at relative rates which are disadvantageous. However, as indicated by curve b in FIG. 4, when the same composite wire is heated to a temperature of about l000F. the initial rate of recrystallization of the wire materials significantly exceeds the rate of intermetallic formation, resulting in enhanced conductivity of the composite wire. In fact, when the composite wire is heated to temperatures between about 800F. and I050F. for periods of between about 0.05 and 1.0 seconds, it is found that significant recrystallization or annealing of the composite wire material can be accomplished before any significant formation of embrittling intermetallic compounds has occurred in the wire. Accordingly it is an important part of this invention to anneal copper-clad aluminum wires at temperatures between 800F. and 1050F. for periods between 0.05 and 1.0 seconds.

It has also been found that, when the annealing times and temperatures for the copper-clad aluminum wire are varied within the limits described, the composite wire can be provided with a variety of advantageous strength and deformation characteristics as desired. In fact, several factors governing the annealing process can be controlled by these means either to selectively anneal only the copper-cladding or to substantially fully anneal both the copper cladding and aluminum core.

That is, the core and cladding of the composite wire 12 can be considered to comprise separate resistances 14a and 16a which are arranged in parallel as indicated in the schematic drawing of FIG. 3. As a result, when electrical current is directed through the composite wire, substantially greater current density is provided in the copper-cladding than in the aluminum core. Where the resistance of the aluminum core is one and one-half times as great as the resistance of the copper cladding, as in a typical composite wire, the heating of the copper cladding will occur substantially faster than the heating of the aluminum core. Similarly, the skin effect inherent in electrical transmission through small conductors also tends to cause greater current density in the cladding of the composite wire. Further, the rate of recrystallization of copper at temperatures between 800F. and 1050F. is found to be substantially greater than the rate of recrystallization of aluminum at these temperatures. Where the wire heating is of very short duration within the limits above described, so that little heating of the core occurs by conduction of heat from the cladding, this means that the copper cladding can be substantially fully annealed before any significant annealing of the wire core occurs. Alternatively, where the wire heating is of longer duration but still within the limits above-described, the copper cladding and aluminum core can each be substantially annealed without the formation of embrittling intermetallics between the core and cladding.

For example, when a copper-clad aluminum wire of 0.45 inch diameter embodying 20 percent copper by volume is moved between contacts 22 and 24 spaced 10 feet apart at a rate of 100 feet per second, and when the rheostat 30 is adjusted to direct amperes at 60 volts through the length of composite wire 12 (so that approximately 0.235 joules of energy are applied to the composite wire per circular mil foot), the copper-cladding on the wire is substantially fully annealed whereas the aluminum wire core remains in substantially hard drawn condition. Joules per circular mil foot are determined according to the following equation:

Amperes X Volts Joules/CMF CM fn/Sec.

The. composite wire annealed in this manner is provided with novel and advantageous strength and deformation characteristics. That is, as indicated by curve 0 in FIG. 5, the composite wire is provided with a tensile strength of 25,000 pounds per square inch (KPSI) and with a yield strength of 17.5 kilograms per square millimeter (Kglmm While the wire displays only very low elongation characteristics as indicated by curve d in FIG. 5, primarily because the core is in hard drawn condition, the wire has excellent bending deformation characteristics and is easily bent for stranding, terminating and the like. This bending flexibility is due to the fact that the most severe stresses occurring during bending of a composite wire occur in the wire cladding, and the cladding of the noted wire is fully annealed. This combination of high tensile strength and particularly high yield strength with good bending deformation characteristics makes the wire extremely well suited for a variety of applications, the wire being comparable in strength and bending deformation properties to monolithic copper wire but displaying a substantially better strength to weight ratio than the all-copper wire. As indicated in FIG. 5, wire having substantially this combination of good tensile and yield strength with good bending properties, is achieved when the power input is maintained within the limits of about 0.235 and 0.245 joules per circular mil foot.

When additional current is applied to the composite wire according to the method of this invention, the copper cladding and aluminum core of the wire are each substantially annealed as indicated by curves c and d in FIG. 5. That is, when between 0.245 and 0.285 joules per circular mil foot energy is applied to the composite wire 12, the tensile and yield strength of the composite wire are reduced to significantly lower levels while the elongation properties of the wire are substantially increased, thereby indicating that both the wire core and cladding are substantially annealed. This substantially fully annealed wire therefore has'strength and deformation characteristics comparable to those of batch annealed composite wires known in the prior art. However, because the process for annealing the composite wire according to the method of this invention is substantially less expensive than batch annealing, the substantially fully annealed composite wire resulting from this improved method is of much lower cost.

Where even greater current is applied to the composite wire by the resistance annealing of this invention, for example when more than about 0.285 joules per circular mil foot are applied to the wire, embrittling intermetallic compounds tend to form at the wire core and cladding interface resulting in increase in tensile and yield strength of the wire and in substantial decrease in the elongation and bending properties of the wire as indicated by curves 0 and din FIG. 5. Wire having such intermetallic compounds therein has poor conductivity and bending characteristics and is of very limited usefulness.

lt should be understood that the annealing method. of this invention can be used with any composite wire or tube having one or more cladding layers where the cladding and core thicknesses are sufficiently uniform to permit resistance annealing. The annealing method is preferably employed in-line in conjunction with a continuous wire drawing operation as illustrated so that the handling of the material required for drawing can also serve for annealing the wire, thereby realizing a considerable economy over batch annealing of drawn wire material.

It can be seen that the novel and improved methods provided by this invention are inexpensive to perform but serve to provide copper-clad aluminum wires with selected strength and deformation characteristics. The methods of this invention also provide composite wires with novel and improved strength and deformation characteristics. It should be understood that although particular embodiments of the composite wire and methods of this invention have been described in detail for the purpose of illustration, this invention includes all modifications and equivalents thereof falling within the scope of the appended claims:

We claim:

1. A method for providing a copper-clad aluminum core wire of selected diameter and selected strength and deformation characteristics comprising the steps of continuously drawing the wire for reducing the wire to said selected diameter while maintaining the thickness of said core and cladding uniform throughout the length of the wire within a tolerance of plus or minus 5 percent of said core and cladding thicknesses, continuously advancing the drawn wire to takeup means, continuously engaging the wire being advanced at spaced locations along the wire and continuously directing electrical current longitudinally through the wire between said locations, regulating the duration of said electrical current between about 0.05 and l second, and regulating said current to furnish between about 0.235 and 0.285 joules per circular mil foot to said length of wire for heating said wire to a temperature at which the rate of recrystallization of at least said copper material substantially exceeds the rate at which intermetallic compounds of said core and cladding materials are formed at said interface for selectively annealing at least said copper cladding without significant formation of embrittling intermetallic compounds between said core and cladding.

2. A method as set forth in claim 1 wherein said current is regulated to furnish between about 0.235 and 0.245 joules per circular mil foot to said wire for selectively annealing said copper cladding without significantly annealing said aluminum core.

3. A method as set forth in claim 1 wherein said current is regulated to furnish between about 0.245 and 0.285 joules per circular mil foot to said wire for substantially annealing said copper cladding and aluminum core.

4. A method as set forth in claim 1 wherein said wire is advanced through a liquid bath after annealing for rapidly cooling said wire.

5. A method for annealing a composite wire having an aluminum core and a copper'cladding metallurgically bonded to said core at an interface between said core and cladding, said core and cladding having thicknesses which are uniform along the length of said wire within a tolerance of plus or minus about 5 percent of said core and cladding thicknesses said method comprising the steps of continuously advancing the wire toward takeup means, continuously engaging the wire being advanced at spaced locations along the wire and directing electrical current longitudinally through the wire between said locations, regulating the intensity of said current for heating said wire to a temperature at which the rate of recrystallization of at least one of said core and cladding materials substantially exceeds the rate at which intermetallic compounds of said materials are formed at said interface, and regulating the duration of said heating for selectively annealing at least said one wire material without significant formation of said intermetallic compounds at said interface. 

